This invention relates to a method for improving the magnetic property of a thin strip of amorphous alloy.
In recent years, remarkable efforts have gone into research toward for the materialization of amorphous metals excelling in mechanical properties such as strength and toughness, chemical properties such as resistance to corrosion, and magnetic properties such as saturation flux density and permeability.
Such amorphous metals are applied to iron cores in transformers of high efficiency for the purpose of taking full advantage of their outstanding magnetic properties. Especially, good promise for future application is shown by materials for magnetic heads to which these amorphous metals are applied with a view to making the most of their high resistance to wear as well as their outstanding magnetic properties. The amorphous metal is produced by fusing a certain genus of metals and/or semimetals compounded at a prescribed proportion of concentrations and cooling the fused mass at a high rate of speed so that the composition does not undergo crystallization. Specifically, a certain method which is generally adopted effects continuous production of the amorphous metal by the steps of reducing to a fused state a mixture using transition metal and noble metal elements such as Fe, Co, Ni, Pb and Au as principal ingredients and additionally incorporating therein semimetals such as B, C, Si and P in a critical proportion of concentrations, spurting the fused mixture onto the rotating surface of a rotary cooling member and thereby allowing the spurted mixture to cool and solidify rapidly. There have been proposed many measures for further improving the outstanding properties of such amorphous alloys. Particularly in the case of amorphous alloys of the kind formulated as magnetic materials for use in magnetic heads and magnetic circuits, there have been proposed a technique (Japanese Patent Disclosure No. 103924/1978) which aims to preclude the thermal deterioration of initial permeability, a phenomenon which the conventional amorphous alloy is designed to undergo upon exposure to heating even at a relatively low temperature, by fixing the concentrations of the component metals thereof within a specific proportion and a technique (Japanese Patent Disclosure No. 43028/1978) which aims to improve the effective permeability of the amorphous alloy by re-heating the alloy up to a temperature falling within a specific range and cooling the heated alloy at a specific rate of temperature decrease.
There has also been proposed a technique (Japanese Patent Publication No. 37133/1975) which, for the purpose of minimizing the phenomenon of magnetic aging (deterioration of magnetic properties with lapse of time) occurring not on amorphous metals but on ordinary steels used as magnetic materials, gives an over-aging treatment to steel plates by imparting stress thereto. As may be inferred from the nature of these techniques, in such crystalline alloys, even if magnetic aging can slightly be restrained, magnetic aging cannot perfectly be eliminated. Still less is it possible to improve the magnetic properties. In fact, such impartment of stress to crystalline alloys encourages occurrence of internal strain and brings about a decline in magnetic properties.
As the result of much research, the inventors have ascertained that an amorphous alloy which is produced in the shape of a thin strip by rapid cooling on the cooling roller produces powerful internal stress and that this internal stress constitutes itself the principal cause for the heavy deterioration in magnetic properties, particularly the property to permit ready magnetization and demagnetization (soft magnetic property). For example, they delivered a paper on the results of their study on the distribution of internal stress present in thin strips of amorphous alloys produced by the quick-cooling method and on the effects of such internal stress upon the magnetic properties at the Scientific Lecture Meeting held on Sept. 20, 1978 by Japan Applied Magnetism Society and at the National Meeting held on Oct. 4, 1978 by Japan Metallurgical Society.
There has been proposed a method for relieving amorphous metals of the internal stress generally liable to impair the soft magnetism by subjecting the amorphous metals to a prolonged thermal treatment at a temperature high enough to effect elimination of the internal stress. The heat treatment continued for a long time at the elevated temperature promotes the embrittlement of the amorphous metals and eventually notably impairs the outstanding mechanical properties owned inherently thereby.
In an effort to improve the magnetic properties of amorphous alloys, strict selection of specific concentrations of component metals in such alloys has been encouraged. Measures proposed in this respect may be roughly grouped under the following two general methods.
(1) Method which comprises subjecting the alloy to a thermal treatment performed at a temperature close to the crystallization transition point for a relatively short period ranging from some tens of minutes to several hours or at a temperature amply lower than the crystallization transition point for a very long period.
(2) Method which comprises combining the treatment of (1) with cooling in the magnetic field.
Because of the high temperatures used for the thermal treatments, these methods deteriorate the mechanical properties of the amorphous alloys and expose such alloys to the thermal treatments for very long periods. Owing to the various disadvantages mentioned above, need has been acutely felt for the development of a novel technique capable of imparting outstanding magnetic properties to amorphous alloys.
An object of this invention is to provide a method for improving the magnetic properties of an amorphous alloy simple by combining the effects of a thermal treatment and the effects of a proper external stress, thereby lowering the temperature used for the thermal treatment, avoiding deterioration of mechanical properties and shortening the period of the thermal treatment.